1 Field of the Invention
The invention relates to a display device comprising a matrix of pixels at the area of crossings of row and column electrodes, each pixel comprising at least a current adjusting circuit based on a memory element, in series with a luminescent element.
2 Description of Related Arts
Such electroluminescence-based display devices are increasingly based on (polymer) semiconducting organic materials. The display devices may either luminesce via segmented pixels (or fixed patterns) but also display by means of a matrix pattern is possible. The adjustment of the pixels via the memory element determines the intensity of the light to be emitted by the pixels. Said adjustment by means of a memory element, in which extra switching elements are used (so-called active drive) finds an increasingly wider application.
Suitable fields of application of the display devices are, for example, mobile telephones, organizers, etc.
A display device of the type described in the opening paragraph is described in PCT WO 99/42983. In said document, the current through a LED is adjusted by means of two TFT transistors per pixel in a matrix of luminescent pixels; to this end, a charge is produced across a capacitor via one of the TFT transistors. This TFT transistor and the capacitor constitute a memory element. After the first TFT transistor has been turned off, the charge of the capacitor determines the current through the second TFT transistor and hence the current through the LED. At a subsequent selection, this is repeated.
In this drive mode, the LED conveys current also during non-selection, which is at the expense of dissipation and results in faster ageing. Moreover, artefacts occur in moving images.
It is, inter alia, an object of the present invention to provide a display device of the type described in the opening paragraph in which the above-mentioned problems occur to a lesser extent. To this end, such a display device is characterized in that the display device comprises at least one independently switchable switch in the current path of the current adjusting circuit and the luminescent element.
By means of the switch (for example, a TFT transistor or a bipolar transistor), the luminescent elements are provided with a current corresponding to the desired luminance. The adjustment of a part of the drive circuit takes place prior to closing of the switch. Parts of this drive circuit (particularly the combination of a capacitor and a transistor associated with the memory element) are used both for pre-adjustment of a drive value and for determining the ultimate current through the luminescent elements. Since the luminescent elements can now convey current for a much shorter time, they are preferably but not necessarily driven in the so-called constant efficiency range. Here, the efficiency of the LED as a function of the diode voltage is practically constant and the quantity of emitted light is practically linearly proportional to the current through the LED. This provides the possibility of accurately adjusting grey values with a high efficiency so that a short drive pulse of the LEDs is sufficient.
In a first embodiment, the display device comprises at least a switch in the current path of the current adjusting circuit and the luminescent element. This, however, requires one switch per pixel and is at the expense of the aperture. For this reason, a preferred embodiment is characterized in that the switch is present between a plurality of luminescent elements and a connection point for an operating voltage.